There is a great need in the semiconductor industry for metrology equipment that can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos.: 4,634,290; 4,646,088; 5,854,710; 5,074,669 and 5,978,074. Each of these patents is incorporated herein by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample that spread out from the pump beam s pot. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. As a result, subsurface features that alter the passage of the thermal and plasma waves have a direct effect on the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
It can be proven theoretically that the overlap and pointing stability of each individual laser is important for accurate results. Therefore, these devices can be quite sensitive to the laser pointing stability. This problem is particularly acute when dealing with semiconductor lasers.
As shown in FIG. 1, and as described in U.S. Pat. No. 5,978,074, cited above, the current Therma-Probe system 100 uses a tracker mechanism to correct for the poor pointing stability of diode lasers. The tracker is a device used to optimize the overlap between the pump and probe lasers in the focusing plane (i.e., the sample). The tracker consists of a long focal length plano-convex lens with two motors to allow movement in both the x and y direction. The tracker is placed in front of the collimated pump laser and adjusts the pump laser output to match the output of the probe laser in the focusing plane.
The following optimization method is then used to correct for the pointing instability. The tracker is scanned first in x-direction and then in y-direction. At each incremental movement the thermal wave signal is recorded on a reference sample. A software routine finds the maximum value of the thermal wave signal at which time the tracker moves to the corresponding position. The tracker scanning function is repeated frequently during operation of the system to improve the pointing stability of the pump laser relative to the probe laser.
A drawback of this methodology is that pump laser effectively chases the probe beam. If the location of the probe beam drifts in a constant direction both lasers may be clipped at the focusing objective. Also, any “structure” in the pump or probe beam profile (which is typical of diode lasers), can lead to false maximums in the thermal wave signal, resulting in an error in pump/probe overlap. Another drawback of the methodology is that the tracker scanning is not performed in a 2-D plane (map) and thus prevents correction of non-symmetrical beams, which is usually the case of diode lasers.
To improve the quality of the probe beam, U.S. Pat. Nos. 6,049,220 and 6,489,801 (both to P. Borden et al.) describe a photothermal system that uses a fiber-coupled infrared probe laser. However, in these patents only one laser (probe) is shown to have a fiber connection that only partially improves the overall system performance.
For these reasons and others there is a need for a system that better optimizes the overlap between pump and probe lasers in modulated reflectance measurement systems. This is particularly important as semiconductor geometries continue to shrink and accurate measurements become increasing difficult to achieve.